Positron Beams and Two-Photon Exchange: The Key to Precision Form Factors

TL;DR

This paper discusses the discrepancy in proton form factor measurements obtained via different methods, explores the role of two-photon exchange effects, and proposes future experiments at DESY and Jefferson Lab to test these effects at higher momentum transfers.

Contribution

It analyzes the potential of upcoming experiments to measure two-photon exchange effects at larger momentum transfers, addressing current theoretical uncertainties.

Findings

Supports existence of small two-photon exchange effects

Highlights limitations of current theoretical treatments

Proposes future measurements at DESY and Jefferson Lab

Abstract

The proton elastic form factor ratio can be measured either via Rosenbluth separation in an unpolarized beam and target experiment, or via the use of polarization degrees of freedom. However, data produced by these two approaches show a discrepancy, increasing with $Q^2$. The proposed explanation of this discrepancy---two-photon exchange---has been tested recently by three experiments. The results support the existence of a small two-photon exchange effect but cannot establish that theoretical treatment at the measured momentum transfers are valid. At larger momentum transfers, theory remains untested. This paper investigates the possibilities of measurements at DESY and Jefferson Lab to measure the effect at larger momentum transfers.